Bipolar transistors are important components in, for example, logic circuits, communication systems, and microwave devices. One type of bipolar transistor is a silicon germanium (SiGe) heterojunction bipolar transistor (HBT). An SiGe HBT can typically handle signals of very high frequencies, e.g., up to several hundred GHz.
Strained SiGe is typically the film of choice for application in NPN HBTs. The SiGe is pseudomorphically grown to match the silicon lattice beneath the SiGe and is, therefore, in a compressively strained state. Subsequent to the pseudomorphic growth process (and in the same reactor) a silicon cap layer can be grown. The silicon cap layer is conventionally doped n-type during the same process using either arsenic (As) or phosphorus (P)—e.g., arsine (AsH3) and phosphine (PH3) are typical dopant gases. The silicon cap layer maintains the SiGe in a strained condition during thermal anneal processes.
After formation of the silicon cap layer, an emitter region is typically formed within an SiGe HBT. The emitter region typically comprises an amorphous, poly-crystalline or mono-crystalline film type. The emitter region is conventionally doped with arsenic (As), phosphorus (P), or another group V element. The emitter region can further include germanium (Ge) to form a polySiGe emitter layer. Conventional SiGe HBTs with a polySiGe emitter layer, however, typically include an oxide interface (or oxide layer) that is formed between the silicon cap layer and the polySiGe emitter layer.
Common device performance metrics of an SiGe HBT include a measurement of a current gain and a breakdown voltage (e.g., the collector-to-emitter breakdown (BVCE0)) associated with the SiGe HBT. Various device parameters that can affect the current gain and/or the breakdown voltage include, for example, a collector current, a base current, emitter resistance, and the like. A common goal of designers is to seek new ways to add device-tuning capability to conventional SiGe HBTs that can affect one or more of these device parameters to reduce current gains and increase breakdown voltages of an SiGe HBT.
Accordingly, what is needed are methods of material engineering that will reduce current gains and increase the breakdown voltages, e.g., the BVCE0, of an SiGe HBT without adverse affect to device speed and power requirements. The present invention addresses such a need.